55 Cancri
55 Cancri is a G8V / M3.5-4V binary, 41 light-years from the sun. As of 2007, the system had 5 planets, and in 2008, the possible sixth and seventh planets were discovered, they are unconfimed. The innermost planet is thought to be a terrestrial "super-Earth" planet, with a mass similar to Neptune, while the outermost planets are thought to be Jovian planets with masses similar to Jupiter. 55 Cancri A is ranked 63rd in the list of top 100 target stars for the NASA Terrestrial Planet Finder mission. Visibility 55 Cancri A has an apparent magnitude of 5.95, making it visible through binoculars. It is just visible to the naked eye under very dark skies. The red dwarf 55 Cancri B is of the 13th magnitude and only visible through a telescope. System In 1997, the discovery of a 51 Pegasi-like planet orbiting 55 Cancri A was announced, together with the planet of Tau Boötis and the inner planet of Upsilon Andromedae. The planet was discovered by measuring the star's radial velocity, which showed a periodicity of around 14.7 days corresponding to a planet at least 78% of the mass of Jupiter. This planet was designated 55 Cancri b, though to distinguish it from the star 55 Cancri B it is occasionally referred to as 55 Cancri Ab. The radial velocity measurements still showed a drift unaccounted-for by this planet, which could be explained by the gravitational influence of a more distant object. In 1998 the discovery of a possible dust disk around 55 Cancri A was announced. Calculations gave the disk radius at least 40 AU, similar to the Kuiper belt in our solar system, with an inclination of 25° with respect to the plane of the sky. However, the discovery could not be verified and was later deemed to be spurious, caused instead by background radiation. After making further radial velocity measurements, a planet orbiting at a distance of around 5 AU was announced in 2002. This planet received the designation 55 Cancri d. At the time of discovery, the planet was thought to be in an orbit of mild eccentricity (close to 0.1), however this value was increased by later measurements. Even after accounting for these two planets, a periodicity at 43 days remained, possibly due to a third planet. Measurements of the star suggested that this was close to the star's rotation period, which raised the possibility that the 43-day signal was caused by stellar activity. This possible planet received the designation 55 Cancri c. In 2004 a Neptune-mass planet designated 55 Cancri e was announced in a 2.8-day orbit. This planet may either be a small gas giant or a large terrestrial planet. The measurements that led to the discovery of this planet also confirmed the existence of 55 Cancri c. In addition, astrometric measurements made by the Hubble Space Telescope led to an estimate of the inclination of the orbit of the outer planet: around 53° with respect to the plane of the sky. In 2005 the existence of planet e was questioned by Jack Wisdom in a reanalysis of the data. According to him, instead of the 2.8-day planet there is a planet with a mass similar to that of Neptune in a 261-day orbit (corresponding to 0.77 AU in distance). This analysis has been partially confirmed in November 2007 – a planet designated 55 Cancri f with half the mass of Saturn was announced in a 260-day orbit, right in 55 Cancri A's habitable zone. The planet itself is not thought to be conducive to life, but hypothetical moons in principle could maintain at least microbial life. There was a METI message sent to 55 Cancri. It was transmitted from Eurasia's largest radar – 70-meter (230-foot) Eupatoria Planetary Radar. The message was named Cosmic Call 2, it was sent on July 6, 2003, and it will arrive at 55 Cancri in May 2044. Category:Stars with planetary systems Category:Cancer Category:G-type stars